An instrument developed by the United States to measure the
rate at which metals and organics corrode when exposed to the
Martian environment is set for launch on Nov. 16 aboard two small
landing stations on the Russian Mars '96 spacecraft.

The instrument, called the Mars Oxidant Experiment (MOx),
was built at NASA's Jet Propulsion Laboratory, Pasadena, CA, and
is part of the expanding U.S.-Russian cooperation effort in space
exploration.

Integration and final testing of the experiment on the
Russian landers, also referred to as "small autonomous stations,"
was completed in late October at the Lavochkin Research and
Production Association in Moscow, where the landers were designed
and assembled, said Mark Herring, manager of the experiment at
JPL. Two of the MOx instruments will fly on the mission, one on
each of the two landers.

"This was an engineering milestone for the U.S. experiment,
culminating a development effort which started in 1992," Herring
said. "In the course of integration on the landers, the U.S. team
was required to take numerous trips to Helsinki and Moscow during
the past year. We've gained valuable experience in the
participation on an international mission."

The goal of the Mars '96 mission is to investigate the
evolution of the Martian atmosphere, surface and interior. The
mission, which will be launched by a Russian Proton launch
vehicle from Baikonur Cosmodrome in Kazakhstan, is designed to
acquire, using a variety of instruments, wide-scale,
comprehensive measurements of the physical and chemical processes
that occur on Mars today and those which took place in the past.

The Mars Oxidant Experiment was developed to further
investigate the presence of a strong oxidizing agent in the
Martian soil which was inferred from the results of the biology
experiments onboard the Viking landers in the mid-1970s.

"We hope MOx will be able to tell us more about the
surprisingly reactive properties of the Martian soil first
detected by the Viking biology experiments and tell us if this
reactivity is the cause of the complete absence of organics in
the surface soil on Mars," said Dr. Christopher McKay, project
scientist at NASA's Ames Research Center, Mountain View, CA.

"If we plan to search for the organic remnants of early life
on Mars with future missions, then we have to understand the
processes that are destroying these organics on the surface so
that we know how deep we have to dig to reach unoxidized
material," he added. "Viking, for instance, dug under a rock as
deep as 11 centimeters (4 inches) but found only oxidized sand."

MOx uses chemical sensor technology originally developed at
the Sandia National Laboratories, Albuquerque, N.M. The
instrument measures the oxidizing power of the Martian soil and
atmosphere using a detector that monitors the change in
reflectivity of a thin chemical film that is exposed to the
Martian environment. The instrument, which weighs only 1.3
kilograms (3 pounds), relies on its own power source, a set of
batteries, to carry out the measurements.

Upon landing and deployment, MOx will operate autonomously,
Herring said, according to a sequence that is programmed into its
internal "read-only memory." While the mission is designed for a
one-year lifetime, the operating life of MOx will be limited by
its battery power source. Depending on the actual conditions on
the surface of Mars, the operating time will be between 80 and
160 days.

"The instrument's sensor head is located on a petal of each
of the two Russian small stations and is comprised of eight
sensor cell assemblies, four of which are designed to contact the
soil and four that will be exposed to the atmosphere," Herring
said. "Within each cell assembly there are six active sensing
sites and six reference sites, for a total of 96 sites.

"The active sites are protected by thin membranes of silicon
nitride, which protect the sensor films from premature
oxidation," he explained. "These membranes will be broken upon
deployment, exposing the active films. The reference sites will
remain permanently sealed. The sensor films have been selected
to provide a broad range of chemical reactions. Each film type
is duplicated in the air and soil cells."

Each of the 96 sensor sites is illuminated by two light-
emitting diodes (LEDs), one operating at a wavelength of 590
nanometers and the other at 870 nanometers. The reflected signal
will be measured by a silicon photodiode detector array. The
sensor sites are coupled to the LEDs and the detector array
through fiber optics.

Data will be stored in the instrument's own internal
computer memory and read out in response to a request from the
small station's data system, Herring said. Command software is
designed so that data acquisition takes priority over
measurements. "If a measurement is in process when telemetry is
requested, it will be halted and then continued after the
telemetry session is complete," he said.

Another feature of the experiment's data transmission
sequencing is its ability to transmit data three times in order
to reduce the data loss associated with various communications
links. During the mission, the experiment team will distribute
calibration data and mission data sets in which data from the
instruments are merged with pertinent mission information.

Also onboard the spacecraft, attached to the MOx electronics
case, is a CD-ROM, entitled "Visions of Mars," produced by The
Planetary Society, Pasadena, CA, which is analogous to the
records carried into space in 1977 by the twin Voyager
spacecraft. The Mars '96 CD-ROM contains a collection of science
fiction stories, sounds and artwork which chronicle humanity's
fascination with Mars over four centuries of human history, 10
alphabets, 17 languages and 26 nations. The collection covers the
earliest references to Mars in science fiction to present day
stories about the red planet. A label pointing to the location of
the CD-ROM is mounted on the outside of the spacecraft and
includes a microdot of 100,000 names of Planetary Society members
and instructions on how to use the CD-ROM.

If launched on time, Mars '96 will reach the orbit of Mars
in mid-September 1997, at about the same time as Mars Global
Surveyor, and land the two small stations and two penetrators on
the surface of the planet. On-time launches of both spacecraft
will enable Mars Global Surveyor to assist in relaying data from
the Russian small stations once its primary mapping mission
begins in March 1998.